The Gornergrat line is not a normal Swiss train line, but a mountain train for tourists. It’s quite expensive, hence the barriers. While you can take this train in a wheelchair, Findelbach station is not accessible because it only serves the hiking trails. On the other side of the station, there is also definitely no way to go around the turnstile: Station Findelbach

According to swisstopo, there is no hiking trail crossing the station, just a path leading to it from either side. I expect the way to the station to be signed on the nearby guideposts, but I’d be surprised if the way to any other destination is signed such that you’d have to cross the station because there are many other options using official trails. Therefore I think access=customers is appropriate for the turnstiles.